Semiconductor laser device

ABSTRACT

A semiconductor laser device includes: active layer; first cladding layer, which is formed on the active layer and is made of (Al X1 Ga 1−X1 ) Z1 In 1−Z1 P (where 0≦X1≦1 and 0&lt;Z1&lt;1) of a first conductivity type; current blocking layer, which is formed on the first cladding layer and is made of (Al Y Ga 1−Y ) Z2 In 1−Z2 P (where 0≦Y≦1 and 0&lt;Z2&lt;1) of a second conductivity type and has striped region; and second cladding layer, which is formed at least in the striped region and is made of (Al X2 Ga 1−X2 ) Z3 In 1−Z3 P (where 0≦X1≦1 and 0&lt;Z3&lt;1) of the first conductivity type. X 1 , X 2  and Y have relationships represented as Y&gt;X1 and Y&gt;X 2 . Saturable absorption region absorbing laser light produced from the active layer is formed in part of the active layer under the current blocking layer.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor laser device,and more particularly relates to a self-sustained pulsation typesemiconductor laser device that emits red laser light.

[0002] A red-light-emitting semiconductor laser is widely used as alight source for a DVD apparatus. Also, a self-sustained pulsation typesemiconductor laser device does not need an external high frequencymodulation circuit for reducing external optical feed back noise andtherefore can be a key device in terms of size and cost reduction.

[0003] Hereinafter, a typical known structure for a self-sustainedpulsation type semiconductor laser device will be described withreference to FIG. 8.

[0004]FIG. 8 illustrates a cross-sectional structure for a knownsemiconductor laser device 10. N-type cladding layer 12 made of ann-type AlGaInP layer; active layer 13 with a multiple quantum wellstructure; first p-type cladding layer 14 made of a p-type AlGaInPlayer; saturable absorption layer 15; and second p-type cladding layer16, which is made of a p-type AlGaInP layer and has a ridge portion, arestacked in this order over an n-type GaAs substrate 11. That is to say,the saturable absorption layer 15 is inserted between the first andsecond p-type cladding layers 14 and 16.

[0005] A current blocking layer 17 of an n-type GaAs layer is formed onthe second p-type cladding layer 16 to cover both sides of the ridgeportion. A contact layer 18 is formed on part of the ridge portion ofthe second p-type cladding layer 16, which is sandwiched by the currentblocking layer 17. And a cap layer 19 of a p-type GaAs layer is formedon the current blocking and contact layers 17 and 18.

[0006] Further, an n-side electrode 20 is formed on the lower surface ofthe n-type GaAs substrate 11, while a p-side electrode 21 is formed onthe upper surface of the cap layer 19.

[0007] In order to fabricate the known semiconductor laser device,n-type AlGaInP layer to be the n-type cladding layer 12; active layer13; p-type AlGaInP layer to be the first cladding layer 14; saturableabsorption layer 15; p-type AlGaInP layer to be the second p-typecladding layer 16; and contact layer 18 are stacked in this order overthe n-type GaAs substrate 11 by a crystal growth process (i.e., firstgrowth process). Thereafter, the second cladding layer 16 and contactlayer 18 are etched and patterned to form a ridge portion. Next, ann-type GaAs layer to be the current blocking layer 17 are selectivelyformed on both sides of the ridge of the second cladding layer 16 byanother crystal growth process (i.e., second growth process).Subsequently, a p-type GaAs layer to be the cap layer 19 is formed onthe contact and current blocking layers 18 and 17 by another crystalgrowth process (i.e., third growth process).

[0008] The distribution 22 of laser light, emitted from the active layer13, is confined in a part of the active layer 13 under the ridgeportion. However, self-sustained pulsation is realized because thesaturable absorption layer 14 exists within the range in which the lightis distributed.

[0009]FIG. 9 illustrates the optical output-current characteristic ofthe known semiconductor laser device. In FIG. 9, curves a, b, c, d and erepresent the characteristics of the semiconductor laser device attemperatures of 20° C., 30° C., 40° C., 50° C. and 60° C., respectively.As can be seen from FIG. 9, non-continuous characteristics resultingform the self-sustained pulsation is observable in the vicinity of thethreshold current. It should be noted that the operating current is 86.6mA when the optical output is 5 mW at room temperature (25° C.).

[0010] However, in the known semiconductor laser device, the currentblocking layer 17 is made of GaAs and therefore absorbs a great deal ofred laser light emitted from the active layer 13. For that reason, theinternal loss at the optical waveguide is as large as about 20 cm⁻¹,thus causing a problem that the operating current of the semiconductorlaser device increases.

[0011] Furthermore, increased heat is generated from the semiconductorlaser device due to the large operating current, and the knownsemiconductor laser device cannot be built in an optical pickupapparatus for a DVD, which is in the highest demand now. As a result,the known semiconductor laser device is not suitable for practical use.

[0012] Moreover, the known self-sustained pulsation type semiconductorlaser device needs to perform three crystal growth processes asdescribed above. Accordingly, the device has a problem that it isdifficult to cut down the cost required.

SUMMARY OF THE INVENTION

[0013] In view of the foregoing, it is a first object of the presentinvention to realize a self-sustained pulsation type semiconductor laserdevice having a low operating current. It is a second object of thepresent invention to get the device fabricated by two crystal growthprocesses.

[0014] To achieve these objects, a semiconductor laser device accordingto the present invention includes: an active layer; a first claddinglayer, which is formed on the active layer and is made of(Al_(X1)Ga_(1−X1))_(Z1)In_(1−Z1)P (where 0≦X1≦1 and 0<Z1<1) of a firstconductivity type; a current blocking layer, which is formed on thefirst cladding layer and is made of (Al_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P(where 0≦Y≦1 and 0<Z2<1) of a second conductivity type and has a stripedregion; and a second cladding layer, which is formed at least in thestriped region and is made of (Al_(X2)Ga_(1−X2))_(Z3)In_(1−Z3)P (where0≦X1≦1 and 0 <Z3<1) of the first conductivity type. X1, X2 and Y haverelationships represented as Y>X1 and Y>X2. A saturable absorptionregion for absorbing laser light produced from the active layer isformed in part of the active layer. The part is located under thecurrent blocking layer.

[0015] In the semiconductor laser device of the present invention, thealuminum mole fraction (Y) of the current blocking layer is greater thanthe aluminum mole fraction (X1) of the first cladding layer or thealuminum mole fraction (X2) of the second cladding layer. Therefore, thebandgap energy of each of the first cladding layer, current blockinglayer and second cladding layer can be made greater than the energycorresponding to the oscillation wavelength of the laser light producedfrom the active layer.

[0016] Thus, the first cladding layer, current blocking layer and secondcladding layer are transparent to the laser light emitted from theactive layer, and it is possible to prevent the laser light from beingabsorbed into the first cladding layer, current blocking layer andsecond cladding layer, or the current blocking layer among other things.As a result, the semiconductor laser device of the present invention canreduce its operating current.

[0017] Also, the current blocking layer is transparent to the laserlight, and the distribution of the laser light emitted from the part ofthe active layer located under the striped region can be easily expandedto other parts of the active layer located under the current blockinglayer. Accordingly, the saturable absorption region for absorbing thelaser light produced from the active layer can be formed in those partsof the active layer located under the current blocking layer. As aresult, the semiconductor laser device of the present invention realizesself-sustained pulsation.

[0018] Further, in this structure, the current blocking layer has thestriped region and the second cladding layer is formed in the stripedregion. Accordingly, only two crystal growth processes are needed, andthe fabrication cost of the semiconductor laser device can be reduced.

[0019] In the semiconductor laser device of the present invention, aneffective refractive index difference between the inside and outside ofthe striped region, which is a difference between first and secondeffective refractive indices, is preferably equal to or greater than2×10⁻³ and equal to or smaller than 5×10⁻³. The first effectiverefractive index is determined by a semiconductor multilayer structureexisting inside the striped region to vertically sandwich the stripedregion therebetween and including the second and first cladding layersand the active layer. The second effective refractive index isdetermined by another semiconductor multilayer structure existingoutside of the striped region to vertically sandwich the striped regiontherebetween and including the current blocking layer, the firstcladding layer and the active layer.

[0020] In that case, the size of the saturable absorption region formedin the active layer can be moderate, and good self-sustained pulsationis obtainable.

[0021] In the semiconductor laser device of the present invention, theactive layer preferably has a quantum well structure formed by stackingmultiple quantum well layers and barrier layers one upon the other, anda total thickness of the quantum well layers is preferably 0.03, μm ormore. In such a case, good self-sustained pulsation can be obtained.

[0022] In the semiconductor laser device of the present invention, thefirst cladding layer preferably has a thickness of 0.10 μm or more and0.45 μm or less. In such a case, good self-sustained pulsation isobtainable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a cross-sectional view of a semiconductor laser deviceaccording to an embodiment of the present invention.

[0024] FIGS. 2(a) through 2(e) illustrate how the semiconductor laserdevice according to an embodiment of the present invention realizesself-sustained pulsation. FIGS. 2(a), 2(b), 2(c), 2(d) and 2(e)represent the range of the distribution of the laser light emitted fromthe active layer, effective refractive index difference, lightdistribution, gain distribution and mode gain distribution,respectively.

[0025]FIG. 3 is a graph representing how the waveform of the opticaloutput changed with time when the semiconductor laser device accordingto an embodiment of the present invention was allowed to oscillate inthe self-sustained manner at room temperature.

[0026]FIG. 4 is a graph representing the optical output-currentcharacteristic of the semiconductor laser device according to anembodiment of the present invention when the device was allowed tooscillate in the self-sustained manner at room temperature.

[0027]FIG. 5(a) is a graph representing a relationship between theeffective refractive index difference and spectral half-width when thesemiconductor laser device according to an embodiment of the presentinvention was allowed to oscillate. FIG. 5(b) represents the spectrum oflaser light oscillating in a longitudinal single mode. And FIG. 5(c)represents the spectrum of laser light oscillating in longitudinalmulti-modes.

[0028]FIG. 6 is a graph representing relationships between the overallthickness of the well layers and spectral half-widths in thesemiconductor laser device according to an embodiment of the presentinvention.

[0029]FIG. 7 is a graph representing a relationship between thethickness of the first p-type cladding layer and highest self-sustainedpulsation temperature in the semiconductor laser device according to anembodiment of the present invention.

[0030]FIG. 8 is a cross-sectional view of a known semiconductor laserdevice.

[0031]FIG. 9 is a graph illustrating the optical output-currentcharacteristics of the known semiconductor laser device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Hereinafter, a semiconductor laser device according to anembodiment of the present invention will be described with reference tothe drawings.

[0033]FIG. 1 illustrates a cross-sectional structure for a semiconductorlaser device 100 according to an embodiment. As shown in FIG. 1, n-typecladding layer 102 made of an n-type AlGaInP layer; active layer 103with a multiple quantum well structure; first p-type cladding layer 104made of p-type (Al_(X1)Ga_(1−X1))_(Z1)In_(1−Z1)P (where 0≦X1≦1 and0<Z1<1); current blocking layer 105, which is made of n-typeAl_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P (where 0≦Y≦1 and 0<Z2<1) and has astriped region 105 a; second p-type cladding layer 106 made of p-type(Al_(X2)Ga_(1−X2))Z3In_(1−Z3)P (where 0≦X1≦1 and 0<Z3<1); and contactlayer 107 made of a p-type GaAs layer are stacked in this order over ann-type GaAs substrate 101. It should be noted that the active layer hasa multiple quantum well structure including eight quantum well layersmade of GaInP and nine barrier layers made of Al_(X)Ga_(1−X)InP (where0≦X≦1). Also, an n-side electrode 108 is formed on the lower surface ofthe n-type GaAs substrate 101, while a p-side electrode 109 is formed onthe upper surface of the contact layer 107.

[0034] A method for fabricating the semiconductor laser device of thisembodiment is as follows.

[0035] First, n-type AlGaInP layer to be the n-type cladding layer 102;multiple GaInP layers (quantum well layers) and multipleAl_(X)Ga_(1−X)InP layers (where 0≦X≦1) (barrier layers), which willtogether make the active layer 103; p-type(Al_(X1)Ga_(1−X1))Z₁In_(1−Z1)P (where 0≦X1≦1 and 0<Z1<1) layer to be thefirst p-type cladding layer 104; and n-type(Al_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P (where 0≦Y≦1 and 0<Z2<1) layer to be thecurrent blocking layer 105 are stacked in this order over the n-typeGaAs substrate 101 by a crystal growth process (i.e., first growthprocess). Thereafter, the n-type (Al_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P ispatterned to form the current blocking layer 105 with the striped region105 a.

[0036] Next, a p-type (Al_(X2)Ga_(1−X2))_(Z3)In_(1−Z3)P (where 0≦X2≦1and 0<Z3<1) layer to be the second p-type cladding layer 106 and ap-type GaAs layer to be the contact layer 107 are stacked in this orderon part of the first p-type cladding layer 104, which is exposed in thestriped region 105 a, and over the current blocking layer 105 by anothercrystal growth process (i.e., second growth process). As a result, thesemiconductor laser device of this embodiment is obtained.

[0037] Accordingly, the semiconductor laser device of this embodimentcan be formed by performing these two crystal growth processes, and thecost can be reduced.

[0038] In the semiconductor laser device of this embodiment, thealuminum mole fractions (X1, Y and X2) have relationships represented asY>X1 and Y>X2 among the p-type (Al_(X1)Ga_(1−X1))_(Z1)In_(1−Z1)P (where0≦X1≦1 and 0<Z1<1) layer to be the first p-type cladding layer 104,n-type (Al_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P (where 0≦Y≦1 and 0<Z2) layer tobe the current blocking layer 105 and p-type(Al_(X2)Ga_(1−X2))_(Z3)In_(1−Z3)P (where 0≦X2≦1 and 0<Z3<1) layer to bethe second p-type cladding layer 106.

[0039] In the semiconductor laser device of this embodiment, thealuminum mole fraction (Y) of the current blocking layer 105 is greaterthan the aluminum mole fraction (X1) of the first p-type cladding layer104 or the aluminum mole fraction (X2) of the second p-type claddinglayer 106. Therefore, the bandgap energy of each of the first and secondp-type cladding layers 104 and 106 and current blocking layer 105 can bemade greater than the energy corresponding to the oscillation wavelengthof the laser light produced from the active layer 103.

[0040] In this manner, the distribution of the laser light, emitted frompart of the active layer 103 under the striped region 105 a, can beeasily expanded to other parts of the active layer 103 under the currentblocking layer 105, because the first and second p-type cladding layers104 and 106 and current blocking layer 105 are transparent to the laserlight emitted from the active layer 103.

[0041] On the other hand, the current injection region of the activelayer 103 is limited to its part under the striped region 105 a by thecurrent blocking layer 105, and no current flows in these parts of theactive layer 103 under the current blocking layer 105. Thus, saturableabsorption regions for absorbing the laser light emitted from the activelayer 103 can be formed. Accordingly, the semiconductor laser device ofthis embodiment realizes self-sustained pulsation.

[0042] Table 1 represents the aluminum mole fractions and thicknesses ofthe contact layer 107, second p-type cladding layer 106, currentblocking layer 105, first p-type cladding layer 104, active layer 103and n-type cladding layer 102, which together make up the semiconductorlaser device of this embodiment. TABLE 1 Al mole Thickness Namefractions (μm) Contact layer — 2.0 Second p-type cladding layer 0.6 1.25Current blocking layer 1.0 0.6 First p-type cladding layer 0.7 0.25Active layer — 0.1374 n-type cladding layer 0.7 1.06

[0043] As shown in Table 1, the aluminum mole fraction (Y=1.0) of thecurrent blocking layer 105 is greater than the aluminum mole fraction(X1=0.7) of the first p-type cladding layer 104. And the aluminum molefraction (Y=1.0) of the current blocking layer 105 is greater than thealuminum mole fraction (X2=0.6) of the second p-type cladding layer 104.It should be noted that, in Table 1, the aluminum mole fraction (Y) ofthe n-type Al_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P to be the current blockinglayer 105 is 1. Alternatively, the aluminum mole fraction does not haveto be 1 but the current blocking layer 105 may contain gallium.

[0044] Also, the active layer 103 has a multiple quantum well structureincluding quantum well layers of GaInP and barrier layers ofAl_(X)Ga_(1−X)InP (where X=0.5). Accordingly, the oscillation wavelengthof the laser light, emitted from the active layer 103, is about 670 nm(corresponding to an energy of 1.85 eV).

[0045] The bandgap energy of each of the first and second p-typecladding layers 104 and 106 and current blocking layer 105 is greaterthan the energy of the laser light emitted from the active layer 103.Therefore, the first and second p-type cladding layers 104 and 106 andcurrent blocking layer 105 are transparent to the laser light emittedfrom the active layer 103. As a result, the internal loss at thewaveguide becomes as small as about several cm⁻³, and the operatingcurrent decreases.

[0046] In the semiconductor laser device of this embodiment, a firsteffective refractive index is determined by a semiconductor multilayerstructure existing inside the striped region 105 a to verticallysandwich the region 105 a therebetween and including the second andfirst p-type cladding layers 106 and 104 and active layer 103. And asecond effective refractive index is determined by another semiconductormultilayer structure existing outside of the striped region 105 a tovertically sandwich the region 105 a therebetween and including thecurrent blocking layer 105, first p-type cladding layer 104 and activelayer 103. An effective refractive index difference between the insideand outside of the striped region is a difference between the first andsecond effective refractive indices and is preferably equal to orgreater than 2×10⁻³ and equal to or smaller than 5×10⁻³.

[0047] Hereinafter, a relationship between an effective refractive indexdifference between the inside and outside of the striped region (whichwill be herein simply called “an effective refractive index difference”)Δn (=n₁-n₂) and self-sustained pulsation will be described. Theeffective refractive index difference is a difference between the firsteffective refractive index n₁, which is determined by the semiconductormultilayer structure existing inside the striped region 105 a tovertically sandwich the region 105 a therebetween and including thesecond and first p-type cladding layers 106 and 104, active layer 103and n-type cladding layer 102, and the second effective refractive indexn₂, which is determined by another semiconductor multilayer structureexisting outside of the striped region 105 a to vertically sandwich theregion 105 a therebetween and including the current blocking layer 106,first p-type cladding layer 104, active layer 103 and n-type claddinglayer 102. It should be noted that, in this embodiment, the effectiverefractive index difference Δn is set to 3.2×10⁻³.

[0048] First, it will be described with reference to FIGS. 2(a) through2(e) how the semiconductor laser device of this embodiment realizesself-sustained pulsation.

[0049] FIGS. 2(a), 2(b), 2(c), 2(d) and 2(e) represent the range of thedistribution 110 of the laser light emitted from the active layer 103,effective refractive index difference, light distribution, gaindistribution and mode gain distribution in the semiconductor laserdevice of this embodiment, respectively.

[0050] In the semiconductor laser device of this embodiment, the currentblocking layer 105 is transparent to the laser light emitted from theactive layer 103 as described above. Accordingly, the laser light ishardly absorbed into the current blocking layer 105, and a considerableproportion of the laser light is transmitted through the currentblocking layer 105. Further, in this embodiment, the effectiverefractive index difference Δn is set to as small a value as 3.2×10⁻³.Accordingly, as shown in FIG. 2(a), the light distribution 110 reachesas far as parts of the first p-type cladding layer 104 and active layer103, which are located under the current blocking layer 105.

[0051] Most of the current injected is confined in the striped region105 a and just a little amount of the current diffuses into the firstp-type cladding layer 104. Therefore, as shown in FIG. 2(d), the currentdistribution (or gain distribution) expands almost no greater than thestriped region 105.

[0052] Accordingly, the mode gain distribution, represented as a productof the light distribution shown in FIG. 2(c) and the gain distributionshown in FIG. 2(d), becomes as shown in FIG. 2(e). That is to say, thelight distribution 110 reaches the outside of the striped region 105 a.However, a gain distributed outside of the striped region 105 a means aloss, and the mode gains distributed in the regions outside of thestriped region 105 a (i.e., hatched regions in FIG. 2(e)) are alsolosses. Accordingly, as shown in FIG. 2(a), saturable absorption regions111 are formed in parts of the active layer 103 on both sides of thestriped region 105 a.

[0053]FIG. 3 represents how the waveform of the optical output changeswith time when the semiconductor laser device of this embodiment isallowed to oscillate in the self-sustained manner at room temperature(25° C.) with its emitting facet uncoated (a state in which no coatinglayer is formed there). As can be seen from the FIG. 3, a stable opticalpulse train is obtained, and satisfactory self-sustained pulsation isrealized. It should be noted that the self-sustained pulsation isrealized at a frequency of 613 MHz.

[0054]FIG. 4 represents a result obtained by measuring the opticaloutput-current characteristics of the semiconductor laser device of thisembodiment when the device was allowed to oscillate in theself-sustained manner at room temperature with its emitting facetuncoated. In FIG. 4, the solid line represents the characteristics ofthe semiconductor laser device of this embodiment while the broken linerepresents the characteristics of a known semiconductor laser device.

[0055] As can be seen from FIG. 4, the known semiconductor laser devicehad an operating current of 86.6 mA at an optical output of 5 mW. Incontrast, the semiconductor laser device of this embodiment had anoperating current of 56.6 mA at the optical output of 5 mW. Accordingly,it was confirmed that the operating current could be reduced to abouttwo-thirds compared to the known device.

[0056] To make the semiconductor laser device oscillate in theself-sustained manner, the saturable absorption regions 111 arenecessary. In this embodiment, the current blocking layer 105 istransparent to the laser light emitted from the active layer 103, andthe effective refractive index difference Δn can be set to a smallvalue. For that reason, the light distribution 110 can be expandedgreatly out of the striped region 105 a , and saturable absorptionregions 111, which are large enough to realize the self-sustainedpulsation, can be obtained.

[0057] Hereinafter, a preferred range of the effective refractive indexdifference Δn for realizing good self-sustained pulsation will bedescribed.

[0058] If the effective refractive index difference Δn is smaller than2×10⁻³, the light distribution 110 expands excessively and the saturableabsorption regions 111 broadens too much. Therefore, no matter how muchcurrent is injected, the absorption of the laser light is not saturated.That is to say, the saturable absorption regions 111 function as a mereabsorber, and the self-sustained pulsation does not occur. Also, if theeffective refractive index difference Δn is smaller than 2×10⁻³, thenthe waveguide structure will be an index-antiguided one. As a result,the transverse mode becomes very unstable.

[0059] On the other hand, if the effective refractive index differenceΔn is greater than 5×10⁻³, the light distribution 110 narrows and thesaturable absorption regions 111 can not function properly. As a result,no self-sustained pulsation occurs.

[0060] Accordingly, to realize good self-sustained pulsation, theeffective refractive index difference Δn needs to be equal to or greaterthan 2×10⁻³ and equal to or smaller than 5×10⁻³.

[0061]FIG. 5(a) shows results obtained by analyzing a relationshipbetween the effective refractive index difference Δn and spectralhalf-width when the laser was allowed to oscillate with the number ofquantum well layers included in the active layer 103 and the thicknessof the first p-type cladding layer 104 changed in the semiconductorlaser device of this embodiment. FIG. 5(b) represents the spectrum ofthe laser light oscillating in a longitudinal single mode while FIG.5(c) represents the spectrum of the laser light oscillating inlongitudinal multi-modes. When oscillating in the self-sustained manner,the laser light has a longitudinal multi-mode spectrum, and the spectralhalf-width becomes about 1 nm. Accordingly, by measuring the spectralhalf-width, it is possible to determine whether self-sustained pulsationis realized or not.

[0062] In FIG. 5(a), ♦ indicates that the number of well layers is sevenand that the thickness of the first cladding layer 104 is 0.15 μm. indicates that the number of well layers is seven and that the thicknessof the first cladding layer 104 is 0.20 μm.

indicates that the number of well layers is seven and that the thicknessof the first cladding layer 104 is 0.25 μm. ◯ indicates that the numberof well layers is eight and that the thickness of the first claddinglayer 104 is 0.20 μm. ⋆ indicates that the number of well layers iseight and that the thickness of the first cladding layer 104 is 0.25 μm.And

indicates that the number of well layers is eight and that the thicknessof the first cladding layer 104 is 0.30 μm.

[0063] Based on FIG. 5(a), it could be confirmed that if the effectiverefractive index difference Δn was equal to or greater than 2.5×10⁻³ andequal to or smaller than 4.2×10⁻³, self-sustained pulsation wasrealized. According to the results shown in FIG. 5(a), where theeffective refractive index difference Δn was equal to or smaller than2.5×10⁻³ and equal to or greater than 4.2×10⁻³, self-sustained pulsationwas not realized. However, if the effective refractive index differenceΔn is 2×10⁻³ or more and 5×10⁻³ or less, self-sustained pulsation isrealizable by optimizing the number of quantum well layers, totalthickness of the quantum well layers in the active layer 103 or thethickness of the first p-type cladding layer 104.

[0064] Hereinafter, a relationship between the total thickness of thequantum well layers in the active layer 103 (overall thickness of thewell layers) and the self-sustained pulsation characteristics will bedescribed.

[0065]FIG. 6 represents relationships between the overall thickness ofthe well layers and spectral half-widths where the effective refractiveindex difference Δn was set to about 3.2×10⁻³. In FIG. 6, ◯ indicate,from the left, six well layers each having a thickness of 0.0053 μm(overall thickness: 0.0053×6 μm), seven well layers each having athickness of 0.0053 μm (overall thickness: 0.0053×7 μm), eight welllayers each having a thickness of 0.0053 μm (overall thickness: 0.0053×8μm) and six well layers each having a thickness of 0.0080 μm (overallthickness: 0.0080×6 μm).

[0066] As can be seen from FIG. 6, if the total thickness of the quantumwell layers is about 0.035 μm or more, self-sustained pulsation isrealized. In other words, it can be seen that self-sustained pulsationis realized by increasing the total thickness of the quantum well layersto secure a sufficient volume for the saturable absorption regions 111.

[0067] In FIG. 6, the effective refractive index difference Δn is set toabout 3.2×10⁻³. However, even if the effective refractive indexdifference Δn is reduced to about 2×10⁻³ to about 3×10⁻³, self-sustainedpulsation also occurs where the total thickness of the quantum welllayers is 0.030 μm or more.

[0068] Hereinafter, a relationship between the thickness of the firstp-type cladding layer 104 and self-sustained pulsation characteristicwill be described.

[0069]FIG. 7 represents a relationship between the thickness of thefirst p-type cladding layer 104 and highest self-sustained pulsationtemperature (i.e., the highest temperature at which self-sustainedpulsation is realized) where the effective refractive index differenceΔn was set to about 3.2 ×10⁻³ and the active layer 103 had a quantumwell structure, in which eight well layers, each having a thickness ofabout 0.0053 μm, were stacked. In FIG. 7, ◯ indicates thatself-sustained pulsation occurred while X indicates that noself-sustained pulsation occurred.

[0070] As can be seen from FIG. 7, if the thickness of the first p-typecladding layer 104 is about 0.1 μm, oscillation is realized only at roomtemperature (25° C.). As the thickness of the first p-type claddinglayer 104 is increased to about 0.3 μm, self-sustained pulsation occurseven at about 80° C.

[0071] On the other hand, if the thickness of the first p-type claddinglayer 104 exceeds about 0.3 μm, the highest self-sustained pulsationtemperature decreases. Until the thickness of the first p-type claddinglayer 104 is about 0.45 μm, self-sustained pulsation is realized.However, if the thickness of the first p-type cladding layer 104 exceedsabout 0.5 μm, self-sustained pulsation is no longer realized.

[0072] Accordingly, to cause the self-sustained pulsation, the thicknessof the first p-type cladding layer 104 is preferably 0.1 μm or more and0.45 μm or less. In order to realize self-sustained pulsation at about60° C. or more, the thickness of the first p-type cladding layer 104 ispreferably set to 0.2 μm or more and 0.4 μm or less.

What is claimed is:
 1. A semiconductor laser device comprising: anactive layer; a first cladding layer, which is formed on the activelayer and is made of (Al_(X1)Ga_(1−X1))_(Z1)In_(1−Z1)P (where 0≦X1≦1 and0<Z1<1) of a first conductivity type; a current blocking layer, which isformed on the first cladding layer and is made ofAl_(Y)Ga_(1−Y))_(Z2)In_(1−Z2)P (where 0≦Y≦1 and 0<Z2<1) of a secondconductivity type and has a striped region; and a second cladding layer,which is formed at least in the striped region and is made of(Al_(X2)Ga_(1−X2))_(Z3)In_(1−Z3)P (where 0≦X≦1 and 0 <Z3<1) of the firstconductivity type, wherein X1, X2 and Y have relationships representedas Y>X1 and Y>X2, and wherein a saturable absorption region forabsorbing laser light produced from the active layer is formed in partof the active layer, the part being located under the current blockinglayer.
 2. A semiconductor laser device according to claim 1, wherein aneffective refractive index difference between the inside and outside ofthe striped region, which is a difference between first and secondeffective refractive indices, is equal to or greater than 2×10⁻³ andequal to or smaller than 5×10⁻³, the first effective refractive indexbeing determined by a semiconductor multilayer structure existing insidethe striped region to vertically sandwich the striped regiontherebetween and including the second and first cladding layers and theactive layer, the second effective refractive index being determined byanother semiconductor multilayer structure existing outside of thestriped region to vertically sandwich the striped region therebetweenand including the current blocking layer, the first cladding layer andthe active layer.
 3. A semiconductor laser device according to claim 1,wherein a bandgap energy of each of the first cladding layer, thecurrent blocking layer and the second cladding layer is greater than anenergy corresponding to an oscillation wavelength of the laser lightproduced from the active layer.
 4. A semiconductor laser deviceaccording to claim 1, wherein the active layer has a quantum wellstructure formed by stacking multiple quantum well layers and barrierlayers one upon the other, and wherein a total thickness of the quantumwell layers is 0.03 μm or more.
 5. A semiconductor laser deviceaccording to claim 1, wherein the first cladding layer has a thicknessof 0.10 μm or more and 0.45 μm or less.